1. Field of Art
This invention relates to surface acoustic wave signal processors, and more particularly to a monolithic surface acoustic wave module having isolated gate Schottky barrier taps thereon.
2. Description of the Prior Art
It has long been known that correlation of frequency modulated and/or phase shift keyed (such as bi-phase) signals, for signature verification, signal to noise improvement, and/or ranging, may be performed in a variety of ways. Signals of any temporal extent may be compared for correlation by multiplication in a product mixer and integration of the output, as in the common, standard, spread spectrum demodulator. However, correlation is achieved only when the two signals are perfectly synchronized within one code chip, which requires an indeterminate number of shifts in relative timing (in increments related to the code chip duration) of a reference signal to find the proper timing for correlation with the intelligence. In real time operations, such as target signature determination and/or detailed ranging in radar, it may be utterly impossible to acquire synchronization in a requisite time frame.
Another known form of correlation is performed in space by means of surface acoustic waves (SAWs). This may be achieved in the well known diode/SAW correlator of the type described in the commonly owned U.S. Pat. No. 4,016,514 to Reeder and Gilden. In such cases, a single correlation, of signal components distributed across all of the taps of a SAW delay line, is performed at one point in time. A convolver which will provide correlation if the reference signal is time inverted, utilizing a meandering-gate, single field effect transducer formed directly on a saw substrate, is disclosed in Spiermann, A.O.W., "Acoustic-Surface-Wave Convolver on Epitaxial Gallium Arsenide", Electronics Letters, Vol. 11, Nos. 25/26, December, 1975, pp. 614, 615. However, spatial correlators of these similar types are limited to processing of signals having a length (temporal extent) equal to the acoustical length (or propagation delay) of the processing device. And, the two waves must achieve code chip synchronization within the propagation delay of the device.
To overcome the shortcomings of time and space correlators, recent innovations have attempted to provide a plurality of contemporaneous temporal correlations, each phased slightly different from the other, so as to be able to handle long codes while at the same time providing a finite limit on the task of synchronization between the reference and the signal. One such device employs nonlinear interaction, between a surface acoustic wave and taps thereof along the interaction region, which product-mixes two waves launched from opposite ends of a surface acoustic wave device. Then, external integration and some form of correlation detection is provided. One such device is described in Menager and Desormiere, "Surface Acoustic Wave Tapped Correlator Using Time Integration", Applied Physics Letters, Vol. 27, No. 1, July 1, 1975, pp. 1, 2. However, as pointed out by Darby and Maines, "Tapped Delay Line Active Correlator: A Neglected Saw Device", IEEE 1975 Ultrasonics Symposium Proceedings, pp. 193-196, there is a great deal of difficulty in providing a circuitry having a component packing density which is commensurate with such a new device. Further, such devices are necessarily hybrid in nature, and therefore subject to mechanical and temperature problems.
The correlation, product mixing and integrating functions of an integrating correlator are combined in a gap-coupled, Schottky diode/lithium niobate integrating correlator structure, disclosed by Ralston et al, "A New Signal-Processing Device, The Integrating Correlator", IEEE 1977 Ultrasonic Symposium Proceedings, pp. 623-628. In this device, platinum/silicon diodes are formed on the surface of a lithium niobate substrate, and a silicon layer, separated from the diodes by an air gap of several thousand angstroms, provides the non-linearity for product mixing, the integration being performed in the diodes. This apparatus has the distinct disadvantage of trying to maintain the critical air gap; it also requires precharging of the diodes and separate interdigital transducers for signal processing and readout. However, spurious signals, filling, and gain/bandwidth limitations are reported for such a device. A partial improvement, thereover, in a monolithic structure, is described by Tuan, H.C., et al, "A New Zinc-Oxide-On-Silicon Monolithic Storage Correlator", IEEE 1977 Ultrasonic Symposium Proceedings, pp. 496-499. This device provides p.sup.+ diffused diodes in an n-type silicon substrate with a zinc oxide overlayer having gold interdigital electroacoustic transducers thereon. However, this device is reported to have critical biasing problems, excessive spurious signals and limited dynamic range of on the order of 25-35 dB. In all of these devices, spreading due to dispersion inherent in dissimilarly-layered devices reduces the accuracy of timing the correlation peak.
It is apparent that in many applications, a monolithic structure, with its attendant mechanical integrity and temperature compatibility is to be desired. Furthermore, the advantage of the multi-correlation process of integrating saw correlators is increased as the number of taps becomes extremely high (hundreds or more), requiring a large number of complete channels of circuitry in monolithic form. Further, maximum utilization of such devices, particularly in real time applications, require the fastest, simplest process of use; that is, with a minimal number of steps and delays in the process of use.